This invention generally relates to multi-layer piezoelectric ceramic transducers. In particular, the invention relates to the design of ultrasound transducers to improve the sensitivity of an ultrasound imaging system.
Acoustic transducers used in ultrasound imaging are constructed of a piezoelectric material whose surfaces are metal coated and connected to a potential (signal) and ground source respectively. This piezoelectric material is typically comprised of a composition of lead zirconate titanate (PZT) ceramic. During operation, a high-frequency electrical waveform is applied to the PZT electrodes, causing a change in ceramic dimension and generating an acoustic pressure wave or pulse. Conversely when an acoustic reflection contacts the surface of the piezoelectric material, it generates a voltage difference across the electrodes that is detected as a receive signal.
Coaxial cables connecting the acoustic transducer to the system used to generate and detect the electrical waveforms, typically possess an electrical impedance of between 50 to 100 ohms. It is desirable that the elements of an acoustic transducer also possess an electrical impedance similar to that of the cable. However the electrical impedance of the transducer element is a function of the piezoelectric material dielectric constant, the geometric area, and thickness. Since the acoustic response and frequency of a piezoelectric element are optimized for specific relationships relating to geometric area and thickness, these parameters cannot be optimized to match the electrical impedance of the cable.
The impedance matching is particularly important for small transducer elements such as those found in a two-dimensional transducer array. Typical linear phased-array transducer elements are rectangular in shape, that is, they are longer in the elevation direction than in azimuthal direction. However, elements for a two-dimensional array may approach a square geometry in which the elevation direction is shortened to match that in azimuth. Therefore, the smaller two-dimensional array elements possess a significantly higher electrical impedance, perhaps greater than 1000 ohms. Meanwhile ultrasound system cables typically possess an impedance of between 50 to 100 ohms. This impedance mismatch greatly reduces electrical efficiency and, hence, element sensitivity. Through the use of a multi-layer transducer stack, the element impedance of a two-dimensional array can be made much closer to that of the cable.
Ultrasound transducers used for medical imaging and non-destructive testing are characterized by two main properties, sensitivity and bandwidth, which are directly correlated to the penetration and resolution of the imaging system. It is well known in the art that multi-layer piezoelectric structures provide a sensitivity enhancement compared to conventional single-layer devices. This occurs because the multi-layer structure reduces the impedance of the piezoelectric ceramic element, e.g., lead zirconate titanate (PZT). Each element is prepared as a multiple of individual ceramic layers connected electrically in parallel but acoustically in series. In this manner, the element still functions acoustically as if it were a solid ceramic while possessing an electrical impedance that is reduced by the square of the number of ceramic layers.
In a multi-layer PZT transducer array, the N (N>1) layers are coupled acoustically in series, so that the ë/2 resonant thickness is t, the stack thickness. When the polarity of an applied voltage matches the poling direction, the piezoelectric material expands in the thickness direction. Since the electrical polarity is the same as the poling direction for each layer, the layers will expand or compress together. For a given applied voltage, the electric field across each layer (thickness t/N) is greater than that for a single-layer transducer (thickness t), resulting in a larger acoustic output. Conversely, the acoustic output of a single-layer transducer can be equaled at a reduced applied voltage. Electrically, the layers are connected in parallel. Compared to a single-layer device, an N-layer device is essentially the sum of N thinner capacitors in parallel. Since the overall thickness of the structure remains constant for a given frequency of operation, the capacitance of the device increases as a function of N2. Correspondingly, the impedance drops as a function of the inverse of N2.
U.S. Pat. No. 6,260,248 discloses a method of forming a multi-layer monolithic piezoelectric actuator by placing electrodes onto green piezoelectric substrates and then co-firing to form a solid multi-layer structure. However, for structures formed by this method, it is difficult to maintain the tolerances necessary for ultrasound transducers since co-firing of the piezoelectric and electrode materials can lead to waviness or non-planarity in the layers. Also, the formation of piezoelectric ceramics with high electromechanical coupling needed for medical ultrasound applications are best done under high-pressure sintering conditions that are not conducive to layered structures of this type. Therefore it is best to form the multi-layer structure from flat, high-quality piezoelectric sheets rather than from co-fired green ceramic substrates.
A second approach disclosed in U.S. Pat. No. 5,381,385 is to form a layered structure of thin piezoelectric layers, each of which possesses a metal electrode on its surface. The array is fabricated by forming holes (or vias) in a stack of piezoelectric material. The vias may be formed by laser or mechanical drilling. However, drilling of ceramics is a difficult feat, particularly so for small holes through thicker ceramic substrates. Low-frequency acoustic transducers possess a ceramic that may be too thick to easily form vias and small element size for higher-frequency transducers requires a high density of vias, which weakens the ceramic structure. In addition, after the vias have been formed, electrical contact needs to be made to the embedded electrode in the buried ceramic layer. This can be difficult to accomplish due to the aspect ratio of the hole unless the via is large in area.
There is a need for simpler methods of manufacturing multi-layer piezoelectric ceramic structures.